The projects proposed are designed to further our understanding of the role that soluble factors play in the regulation of the immune response. The soluble factor, B cell maturation factor (BMF), is secreted from T cells and affects the functional state of normal small resting B cells as well as some tumor lines. When the cell line 70Z/3 is exposed to BMF, it responds by beginning to synthesize light chain and also it begins to express surface IgM. Normally 70Z/3 expresses no surface IgM. When the cell line WEHI-279.1 is exposed to BMF, it responds by altering its pattern of synthesis for mu heavy chain, going from a state where it primarily synthesizes membrane mu to a state where it predominantly makes secreted mu. The cell also begins to secrete large amounts of IgM. In a similar manner, small resting B cells, when exposed to BMF, begin to secrete immunoglobulin. Our first specific aim is to clone a cDNA which encodes the gene for BMF. We have developed a bioassay to follow BMF activity and will utilize mRNA isolated from the T cell line EL4 as a source of RNA for the cDNA preparation. The cDNA will then be used to synthesize larger quantitaties of BMF for functional studies. The second specific aim is to study the mechanism by which BMF is able to shift the synthesis of membrane mu to secreted mu in the cell line WEHI-279.1. This change has its origins in the alteration of some aspect of RNA processing. How this occurs we do not origins in the alteration of some aspect of RNA processing. How this occurs we do not know. We propose to utilize recently developed transfection procedures, together with in vitro mutagenesis techniques, to determine what sequence(s) is important in the genomic regions flanking the mu gene for this response to occur. This may lead to a better understanding of how RNA processing occurs in general. The third specific aim is to identify the cellular receptor for BMF. Using a series of monoclonal antibodies against 70Z/3 cells. In this manner, we can better integrate the knowledge of this receptor-ligand interaction, with the signal the receptor sends to the immunoglobulin genes for activation.